1. Field of the Invention
The present invention relates to a technique for re-insulating copper conductors having cracked and/or damaged insulation thereon.
2. Description of the Prior Art
There presently exists in telephone company outside plant installations a large amount of plastic insulated copper wire. Over a period of years the insulation on the wire may become cracked due to high temperatures, the presence of oxygen, and other environmental factors. In addition, it is frequently necessary to gain access to telephone conductors for testing purposes. This can result in damage to the insulation on conductors, especially in terminal housings and other access points. To protect and ensure the viability of this investment, restorative materials have been developed for repairing insulation on copper conductors.
Prior art restorative materials have included lacquers, typically applied in the form of a spray. This is in contrast to the frequent prior art use of varnishes for the original insulation of conductors in transformers, motors, etc. Varnishes are also used to preserve wood and other materials exposed to the atmosphere. However, varnishes are typically cured for relatively long periods, often at elevated temperatures, leading workers in the art to choose lacquers for restorative purposes. A lacquer is a material that dries due to the evaporation of a solvent, leaving behind a solid material. For this purpose, typically about a 3 weight percent polyurethane solid has been dissolved in 97 weight percent solvent. More recently, improved insulation sprays have comprised antioxidants to prolong the life of the insulation, typically polyethylene. The resulting insulation sprays typically comprise about 6 weight percent polyurethane and antioxidants in a solvent base. These materials dry to a film upon application in less than ten minutes under typical ambient conditions. However, these restorative materials have not proven entirely satisfactory, for a variety of reasons. In particular, the increase in service life of the reinsulated conductor has not been as great as desired, with cracks being developed in the reinsulated conductor after a few years time.
A variety of attributes are required for an effective reinsulation technique. A restorative material is desirably a material that is safe to handle, low in toxicity, and has the ability to reinsulate exposed copper conductors. It desirably can be delivered with a simple delivery system and should have good flow properties, which implies low viscosity, and it should be applicable over a wide temperature and humidity range. Furthermore, the system should not contain a solvent that can stress crack the plastic material present in associated connectors, plugs and/or terminal blocks. It should be non-corrosive, non-conductive, have low flammability, and be non-hygroscopic (i.e., does not absorb water). Finally, a reinsulation material should not unduly restrict subsequent re-entry of the wire bundle, but should have relatively low tear strength so that access to the copper conductor can be obtained in the future.